Ok, the SB600 let's you do +/- 3 stops of compensation if I remember rightly - 1 will at a guess give you something natural looking , full flash always looks a bit over bright to me at least on stuff like this.
The challenge is to freeze the movement - you rain drops will move the flower when they hit so a relatively fast shutter speed is needed - can't remember the nikon fastest shutter sync , it's probably 1/200 or so plus if you want the whole flower in focus I'd guess an aperture of f11 or maybe f16 with the macro lens would give you good results. Getting movement trails will work with slower shutter speeds a guess would be to start at about 1/2 second and play from there - you might need to back the flash off a little more for that - again a little trial and error will help you get a feel for it.
Trial and error will also be key for getting a drop hitting the flower, use a cable release if you have one and personally I would use single shot and try and time/judge the release with the drops, machine gunning away might work but I always seem to get better results with a more manual approach - but that's just my preference so don't take it as a rule.
Trying to think if I still have some similar shots I did a few years back in similar fashion, if I can find them I'll post them up here somewhere but by then I am sure you will have had chance to play.
Oh one last thing, I'm mostly a landscape photographer these days and whenever there is water around you can guarantee it will find your lens - so keep a lens cloth or two handy and keep an eye on the front of the lens - nothing worse than getting the perfect shot and then finding a dirty great water mark on it!! Been there done that...
Have fun
Andrew
Crikey - I think ole Bryan needs to sort out his hair style - seems to be still stuck in the 80's.... 
